Tag Archives: projectile motion

The Negative 16 Problems and Educational Romanticism

I came up with a good activity that allowed me to wrap up quadratics with a negative 16s application. (Note: I’m pretty sure that deriving the algorithm involves calculus and anyway, was way beyond the scope of what I wanted to do, which was reinforce their understanding of quadratics with an interesting application.) As you read, keep in mind: many worksheets with lots of practice on binomial multiplication, factoring, simpler models, function operations, converting quadratics from one form to another, completing the square (argghh) preceded this activity. We drilled, baby.

I told the kids to get out their primary quadratics handout:


Then I showed two model rocket launches with onboard camera (chosen at random from youtube).

After the video, I toss a whiteboard marker straight up and caught it. Then I raised my hand and drop the marker.

“So the same basic equation affects the paths of this marker and those rockets–and it’s quadratic. What properties might affect—or be affected by—a projectile being launched into the air?”

The kids generated a list quickly; I restated a couple of them.


Alexandra: “What about distance?”

I pretended to throw the marker directly at Josh, who ducked. Then I aimed it again, but this time angling towards the ceiling. “Why didn’t Josh duck the second time?”

“You wouldn’t have hit him.”

“How do you know?”

“Um. Your arm changed…angles?”

“Excellent. Distance calculations require horizontal angles, which involves trigonometry, which happens next year. So distance isn’t part of this model, which assumes the projectile is launched straight….”


“What about wind and weather?” from Mark.

“We’re ignoring them for now.”

“So they’re not important?”

“Not at all. Any of you watch The Challenger Disaster on the Science Channel?”

Brad snickered. “Yeah, I’m a big fan of the Science Channel.”

“Well, about 27 years ago, the space shuttle Challenger exploded 70 some seconds after launch, killing everyone on board when it crashed back to earth.” Silence.

“The one that killed the teacher?”

“Yes. The movie—which is very good—shows how one man, Richard Feynman, made sure the cause was made public. A piece of plastic tubing was supposed to squeeze open and closed—except, it turns out, the tubing didn’t operate well when it was really cold. The launch took place in Florida. Not a place for cold. Except it was January, and very cold that day. The tubing, called O-ring, compressed—but didn’t reopen. It stayed closed. That, coupled with really intense winds, led to the explosion.”

“A tube caused the crash?”

“Pretty much, yes. Now, that story tells us to sweat the small stuff in rocket launches, but we’re not going to sweat the small stuff with this equation for rocket launches! We don’t have to worry about wind factors or weather.”

“Then how can it be a good model?” from Mark, again.

“Think of it like a stick figure modeling a human being but leaving out a lot. It’sstill a useful model, particularly if you’re me and can’t draw anything but stick figures.”

So then we went through parameters vs. variables: Parameters like (h,k) that are specific to each equation, constant for that model. Variables–the x and y–change within the equation.

“So Initial Height is a parameter,” Mark is way ahead.

Nikhil: “But rocket height will change all the time, so it’s a variable.”

Alissa: “Velocity would change throughout, wouldn’t it?”

“But velocity changes because of gravity. So how do you calculate that?” said Brad.

“I’m not an expert on this; I just play one for math class. What we calculate with is the initial velocity, as it begins the journey. So it’s a parameter, not a variable.”

“But how do you find the initial velocity? Can you use a radar gun?”

“Great question, and I have no idea. So let’s look at a situation where you’ll have to find the velocity without a radar gun. Here’s an actual—well, a pretend actual—situation.”neg16question

“Use the information here to create the quadratic equation that models the rocket’s height. In your notes, you have all the different equation formats we’ve worked with. But you don’t have all the information for any one form. Identify what information you’ve been given, and start building three equations by adding in your known parameters. Then see what you can add based on your knowledge of the parabola. There are a number of different ways to solve this problem, but I’m going to give you one hint: you might want to start with a. Off you go.”

And by golly, off they went.

As releases go, this day was epic. The kids worked around the room, in groups of four, on whiteboards. And they just attacked the problem. With determination and resolve. With varying levels of skill.

In an hour of awesomeness here is the best part, from the weakest group, about 10 minutes after I let them go. Look. No, really LOOK!


See negative 2.5 over 2? They are trying to find the vertex. They’ve taken the time to the ground (5 seconds) and taken half of it and then stopped. They were going to use the equation to find a, but got stuck. They also identified a zero, which they’ve got backwards (0,5), and are clearly wondering if (0,4) is a zero, too.

But Ed, you’re saying, they’ve got it all wrong. They’ve taken half of the wrong number, and plugged that—what they think is the vertex—into the wrong parameter in the vertex algorithm.. That’s totally wrong. And not only do they have a zero backwards, but what the hell is (0,4) doing in there?

And I say you are missing the point. I never once mentioned the vertex algorithm (negative b over 2a). I never once mentioned zeros. I didn’t even describe the task as creating an equation from points. Yet my weakest group has figured out that c is the initial height, that they can find the vertex and maybe the zeroes. They are applying their knowledge of parabolas in an entirely different form, trying to make sense of physical data with their existing knowledge. Never mind the second half—they have knowledge of parabolas! They are applying that knowledge! And they are on the right track!

Even better was the conversation when I came by:

“Hey, great start. Where’d the -2.5 come from?”

“It’s part of the vertex. But we have to find a, and we don’t know the other value.”

“But where’d you get 2.5 from?”

“It’s halfway from 5.”

Suddenly Janice got it.

“Omigod–this IS the vertex! 144 is y! 2.5 is x! We can use the vertex form and (h,k)!!”

The football player: “Does it matter if it doesn’t start from the ground?”

Me: “Good question. You might want to think about any other point I gave you.”

I went away and let them chew on that; a few minutes later the football player came running up to me: “It’s 2!” and damned if they hadn’t solved for a the next time I came by.

Here’s one of the two top groups, at about the same time. (Blurry because they were in the deep background of another picture). They’d figured out the vertex and were discussing the best way to find b.


Mark was staring at the board. “How come, if we’re ignoring all the small stuff, the rocket won’t come straight back down? Why are you sure it’s not coming back to the roof?”

“Oh, it could, I suppose. Let me see if I can find you a better answer.” He moved away, when I was struck by a thought. “Hey….doesn’t the earth move? I mean yes, the earth moves. Wouldn’t that put the rocket down in a different place?”

“Is that it?”
“Aren’t you taking physics? Go ask your teacher. Great questions.”

I suggested taking a look at the factored form to find b but they did me one better by using “negative b over 2a” again and solving for b (which I hadn’t thought of), leading to Mark’s insight “Wait–the velocity is always 32 times the seconds to max height!”

The other kids had all figured out the significance of the vertex form, and were all debating whether it was 2.5 or 2 seconds, generally calling me over to referee.

One group of four boys, two Hispanics, one black, one Asian (Indian), all excellent students, took forever to get started, arguing ferociously over the vertex question for 10 minutes before I checked on them to see why they were calling each other “racist” (they were kidding, mostly). I had to chastise the winners for unseemly gloating. Hysterical, really, to see alpha males in action over a math problem. Their nearly-blank board, which I photographed as a rebuke:


The weaker group made even more progress (see the corrections) and the group to their left, middling ability, in red, was using standard equation with a and c to find b:

My other top group used the same method, and had the best writeup:

Best artwork had the model wrong, but the math mostly right:

  • All but one group had figured out they wanted to use vertex form for the starting point.
  • All but one group had kids in it that realized the significance of the 80 foot mark (the mirror point of the initial height)
  • All the groups figured out the significance of five seconds.
  • All the groups were able to solve for both a and b of the standard form equation.
  • The top three groups worked backwards to find the “fake” zero.
  • Two groups used the vertex algorithm to find b.
  • All the groups figured out that b had to be the velocity.

So then, after they figured it all out, I gave them the algorithm:

h(t)=-16t2 + v0t + s0.

Then I gave them Felix Baumgartner, the ultimate in a negative 16 problem.

And….AND!!!! The next day they remembered it all, jumping into this problem without complaint:projmotfollowup

Charles Murray retweeted my why not that essay, saying that I was the opposite of an educational romantic, and I don’t disagree. But he’s also tweeted that I’m a masochist for sticking it out—implying, I think, that working with kids who can’t genuinely understand the material must be a sad and hopeless task. (and if he’s not making that point, others have.) I noticed a similar line of thought in this nature/nurture essay by Tom Bennett says teachers would not write off a child with low grades as destined to stack shelves –implication that stacking shelves is a destiny unworthy of education.

The flip side of that reasoning looks like this: Why should only some students have access to a rich, demanding curriculum and this twitter conversation predicated on the assumption that low income kids get boring curricula with no rigor and low expectations.

Both mindsets have the same premise: education’s purpose is to improve kids’ academic ability, that education without improvement is soulless drudgery, whether cause or effect. One group says if you know kids can’t improve, what a dreary life teaching is. The other group says dreary teaching with low expectations is what causes the low scores—engage kids, better achievement. Both mindsets rely on the assumption that education is improvement.

Is it?

Suppose that in six months my weakest kids’ test scores are identical to the kids who doodled or slept through a boring lecture on the same material. Assume this lesson does nothing to increase their intrinsic motivation to learn math. Assume that some of the kids end up working the night shift at 7-11. Understand that I do make these assumptions.

Are the kids in my class better off for the experience? Was there value in the lesson itself, in the culmination of all those worksheets that gave them the basis to take on the challenge, in the success of their math in that moment? Is it worth educating kids if they don’t increase their abilities?

I believe the answer is yes.

Mine is not in any way a dreary task but an intellectual challenge: convince unmotivated students to take on advanced math—ideally, to internalize the knowledge for later recall. If not, I want them to have a memory of success, of achievement—not a false belief, not one that says “I’m great at math” but one that says “It’s worth a try”. Not miracles. Just better.

I would prefer an educational policy that set more realistic goals, gave kids more hope of actual mastery. But this will do in the meantime.

I have no evidence that my approach is superior, that lowering expectations but increasing engagement and effort is a better approach. I rely on faith. And so, I’m not entirely sure that I’m not an educational romantic.

Besides. It’s fun.